Process for cracking to light olefins

ABSTRACT

A process for catalytic production of olefins comprises contacting a first hydrocarbon stream and a first stream of fluid catalyst in a first riser to produce a first cracked product stream and a spent catalyst stream. The first cracked product stream is separated in a main column. An overhead stream from the main column is separated into a second hydrocarbon stream. The second hydrocarbon stream is contacted with a second stream of fluid catalyst in a second riser to produce a second cracked product stream and a first stream of cool catalyst. A third hydrocarbon stream is obtained from the overhead stream and/or from the second cracked product stream. The third hydrocarbon stream is contacted with a third stream of fluid catalyst in a third riser to produce a third cracked product stream and a second stream of cool catalyst.

FIELD

The field is the reaction of feed with fluid catalyst. The fieldparticularly relates to an FCC process to produce light olefins withmultiple reactors.

BACKGROUND

Catalytic cracking can create a variety of products from largerhydrocarbons. Often, a feed of a heavier hydrocarbon, such as a vacuumgas oil, is provided to a catalytic cracking reactor, such as a fluidcatalytic cracking (FCC) reactor. Various products may be produced fromsuch a system, including a gasoline product and/or light product such aspropylene and/or ethylene.

In such systems, a single reactor or a dual reactor can be utilized.Although additional capital costs may be incurred by using a dualreactor system, one of the reactors can be operated to tailor conditionsfor maximizing products, such as light olefins including propyleneand/or ethylene.

It can often be advantageous to maximize yield of a product in one ofthe reactors. Additionally, there may be a desire to maximize theproduction of a product from one reactor that can be recycled back tothe other reactor to produce a desired product, such as propylene.

Moreover, some dual reactor systems utilize a mixture of catalysts, suchas a larger pore catalyst and a smaller pore catalyst. In someinstances, the proportion of smaller pore catalysts limit the productionof desired light olefins. Consequently, it typically would be beneficialto segregate the catalysts for controlling the reaction thereof.

Thus, there can be a desire to provide a reactor system for catalyticcracking that may maximize operation conditions for maximizing propyleneproduct.

BRIEF SUMMARY

A process for catalytic production of olefins comprises contacting afirst hydrocarbon stream and a first stream of fluid catalyst in a firstriser to produce a first cracked product stream and a spent catalyststream. The first cracked product stream is separated in a main column.An overhead stream from the main column is separated into a secondhydrocarbon stream. The second hydrocarbon stream is contacted with asecond stream of fluid catalyst in a second riser to produce a secondcracked product stream and a first stream of cool catalyst. A thirdhydrocarbon stream is obtained from the overhead stream and/or from thesecond cracked product stream. The third hydrocarbon stream is contactedwith a third stream of fluid catalyst in a third riser to produce athird cracked product stream and a second stream of cool catalyst.

Additional details and embodiments of the invention will become apparentfrom the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The FIG. 1 s a sectional, elevational of the process and apparatus ofthe present disclosure.

Definitions

The term “downstream communication” means that at least a portion offluid flowing to the subject in downstream communication may operativelyflow from the object with which it fluidly communicates.

The term “upstream communication” means that at least a portion of thefluid flowing from the subject in upstream communication may operativelyflow to the object with which it fluidly communicates.

The term “direct communication” means that fluid flow from the upstreamcomponent enters the downstream component without passing through anyother intervening vessel.

The term “indirect communication” means that fluid flow from theupstream component enters the downstream component after passing throughan intervening vessel.

The term “bypass” means that the object is out of downstreamcommunication with a bypassing subject at least to the extent ofbypassing.

As used herein, the term “predominant” or “predominate” means greaterthan 50%, suitably greater than 75% and preferably greater than 90%.

The term “column” means a distillation column or columns for separatingone or more components of different volatilities. Unless otherwiseindicated, each column includes a condenser on an overhead of the columnto condense and reflux a portion of an overhead stream back to the topof the column and a reboiler at a bottom of the column to vaporize andsend a portion of a bottoms stream back to the bottom of the column.Feeds to the columns may be preheated. The top pressure is the pressureof the overhead vapor at the vapor outlet of the column. The bottomtemperature is the liquid bottom outlet temperature. Overhead lines andbottoms lines refer to the net lines from the column downstream of anyreflux or reboil to the column. Stripper columns may omit a reboiler ata bottom of the column and instead provide heating requirements andseparation impetus from a fluidized inert media such as steam. Strippingcolumns typically feed a top tray and take main product from the bottom.

As used herein, the term “separator” means a vessel which has an inletand at least an overhead vapor outlet and a bottoms liquid outlet andmay also have an aqueous stream outlet from a boot. A flash drum is atype of separator which may be in downstream communication with aseparator that may be operated at higher pressure.

As used herein, the term “boiling point temperature” means atmosphericequivalent boiling point (AEBP) as calculated from the observed boilingtemperature and the distillation pressure, as calculated using theequations furnished in ASTM D1160 appendix A7 entitled “Practice forConverting Observed Vapor Temperatures to Atmospheric EquivalentTemperatures”.

As used herein, the term “True Boiling Point” (TBP) means a test methodfor determining the boiling point of a material which corresponds toASTM D-2892 for the production of a liquefied gas, distillate fractions,and residuum of standardized quality on which analytical data can beobtained, and the determination of yields of the above fractions by bothmass and volume from which a graph of temperature versus mass %distilled is produced using fifteen theoretical plates in a column witha 5:1 reflux ratio.

As used herein, “pitch” means the hydrocarbon material boiling aboveabout 524° C. (975° F.) AEBP as determined by any standard gaschromatographic simulated distillation method such as ASTM D2887, D6352or D7169, all of which are used by the petroleum industry.

As used herein, the term “T5” or “T95” means the temperature at which 5mass percent or 95 mass percent, as the case may be, respectively, ofthe sample boils using ASTM D-86 or TBP.

As used herein, the term “initial boiling point” (IBP) means thetemperature at which the sample begins to boil using ASTM D-7169, ASTMD-86 or TBP, as the case may be.

As used herein, the term “end point” (EP) means the temperature at whichthe sample has all boiled off using ASTM D-7169, ASTM D-86 or TBP, asthe case may be.

As used herein, “vacuum gas oil” means a hydrocarbon material having anIBP of at least about 232° C. (450° F.), a T5 of between about 288° C.(550° F.) and about 392° C. (700° F.), typically no more than about 343°C. (650° F.), a T95 between about 510° C. (950° F.) and about 570° C.(1058° F.) and, or an EP of no more than about 626° C. (1158° F.)prepared by vacuum fractionation of atmospheric residue as determined byany standard gas chromatographic simulated distillation method such asASTM D2887, D6352 or D7169, all of which are used by the petroleumindustry.

As used herein, “atmospheric residue” means a hydrocarbon materialhaving an IBP of at least about 232° C. (450° F.), a T5 of between about288° C. (550° F.) and about 392° C. (700° F.), typically no more thanabout 343° C. (650° F.), and a T95 between about 510° C. (950° F.) andabout 700° C. (1292° F.) obtained from the bottoms of an atmosphericcrude distillation column.

As used herein, “vacuum residuum” means hydrocarbon material boilingwith an IBP of at least about 500° C. (932° F.).

DETAILED DESCRIPTION

We have found that twice cracked naphtha or once or twice cracked C4hydrocarbons can still be cracked to generate more propylene. However,conditions should be tailored for cracking twice cracked naphtha or C4hydrocarbons to maximize propylene production. We propose a third riserto provide the favorable reaction conditions to maximize the yield ofpropylene.

Now turning to the FIGURE, wherein like numerals designate likecomponents, a process and apparatus generally includes an FCC unitsection 6 and a product recovery section 8. The FCC unit section 6includes a first FCC reactor 10 comprising a first reactor unit 12 and acatalyst regenerator 14. Process conditions in the first FCC reactor 10may include a cracking reaction temperature of about 400° to about 600°C., preferably about 538° C. to about 593° C. at the reactor outlet, anda catalyst regeneration temperature of about 500° to about 900° C. Boththe cracking and regeneration occur at an absolute pressure betweenabout 100 kPa (14 psia) to about 650 kPa (94 psia), preferably betweenabout 140 kPa (20 psia) to about 450 kPa (65 psia).

FIG. 1 shows a first FCC reactor vessel 12 in which a first hydrocarbonfeedstock in line 15 through a distributor 16 is contacted with a firststream of fluid catalyst entering from a regenerated catalyst standpipe18 and a recirculation catalyst standpipe 19. The first hydrocarbonfeedstock may comprise vacuum gas oil, atmospheric resid, deasphaltedoil, vacuum resid or any other stream processed in a conventional FCCunit.

The catalyst can be a single catalyst or a mixture of differentcatalysts. Usually, the catalyst includes two components or catalysts,namely a first component or catalyst, and a second component orcatalyst. Such a catalyst mixture is disclosed in, e.g., U.S. Pat. No.7,312,370 B2. Generally, the first component may include any of thewell-known catalysts that are used in the art of FCC, such as an activeamorphous clay-type catalyst and/or a high activity, crystallinemolecular sieve. Zeolites may be used as molecular sieves in FCCprocesses. Preferably, the first component includes a large porezeolite, such as a Y-type zeolite, an active alumina material, a bindermaterial, including either silica or alumina, and an inert filler suchas kaolin.

Typically, the zeolitic molecular sieves appropriate for the firstcomponent have a large average pore size. Usually, molecular sieves witha large pore size have pores with openings of greater than about 0.7 nmin effective diameter defined by greater than about 10, and typicallyabout 12, member rings. Pore Size Indices of large pores can be aboveabout 31. Suitable large pore zeolite components may include syntheticzeolites such as X and Y zeolites, mordenite and faujasite. A portion ofthe first component, such as the zeolite, can have any suitable amountof a rare earth metal or rare earth metal oxide.

The second component may include a medium or smaller pore zeolitecatalyst, such as a MFI zeolite, as exemplified by at least one ofZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38, ZSM-48, and other similarmaterials. Other suitable medium or smaller pore zeolites includeferrierite, and erionite. Preferably, the second component is a mediumor small pore zeolite dispersed on a matrix including a binder materialsuch as silica or alumina and an inert filler material such as kaolin.The second component may also include some other active material such asBeta zeolite. These compositions may have a crystalline zeolite contentof about 10 to about 50 wt % or more, and a matrix material content ofabout 50 to about 90 wt %. Components containing about 40 wt %crystalline zeolite material are preferred, and those with greatercrystalline zeolite content may be used. Generally, medium and smallerpore zeolites are characterized by having an effective pore openingdiameter of less than or equal to about nm, rings of about 10 or fewermembers, and a Pore Size Index of less than about 31.

The total catalyst mixture in the first FCC reactor 12 may contain about1 to about 25 wt % of the second component, namely a medium to smallpore crystalline zeolite with greater than or equal to about 1.75 wt %of the second component being preferred. The first component maycomprise the balance of the catalyst composition. In some preferredembodiments, the relative proportions of the first and second componentsin the mixture may not substantially vary throughout the first FCCreactor 12. The high concentration of the medium or small pore zeoliteas the second component of the catalyst mixture can improve selectivityto light olefins. In one exemplary embodiment, the second component canbe a ZSM-5 zeolite and the mixture can include about 4 to about 10 wt %ZSM-5 zeolite excluding any other components, such as binder and/orfiller.

Preferably, at least one of the first and/or second catalysts is an MFIzeolite having a silicon to aluminum ratio greater than about 15,preferably greater than about 75. In one exemplary embodiment, thesilicon to aluminum ratio can be about 15:1 to about 35:1.

The contacting may occur in a narrow first riser 20, extending upwardlyto the bottom of a first reactor vessel 22. The contacting of the firsthydrocarbon feedstock and the first stream of fluid catalyst isfluidized by gas such as steam from a fluidizing distributor 24. In anembodiment, heat from the catalyst vaporizes the first hydrocarbonfeedstock, and the first hydrocarbon feedstock is thereafter cracked toa first cracked product stream of lighter molecular weight in thepresence of the first catalyst stream as both are transferred up theriser 20 into the reactor vessel 22 providing a first mixture ofcatalyst and product gases.

The pressure in the first riser 20 may be about 200 kPa (29 psia) toabout 450 kPa (65 psia), but it could be lower. A steam rate of about 3to about 7 wt % of the first hydrocarbon feedstock is added to the firstriser 20. Inevitable side reactions occur in the first riser 20 leavingcoke deposits on the catalyst that lower catalyst activity to provide aspent catalyst stream. The first cracked product stream in the firstmixture of catalyst and product gases is thereafter separated from thespent catalyst stream using cyclonic separators which may include one ortwo stages of cyclones 62 in the reactor vessel 22. A gaseous, firstcracked product stream exits the reactor vessel 22 through a firstproduct outlet 31 to line 32 for transport to the downstream productrecovery section 8.

The spent or coked catalyst requires regeneration for further use. Thespent catalyst stream, after separation from the first cracked productstream by means of a disengagement device 54 in a first disengagementchamber 56, falls into a stripping section 34 where steam is injectedthrough a distributor 35 to purge any residual hydrocarbon vapor. Afterthe stripping operation, the stripped coked catalyst is carried to thecatalyst regenerator 14 through a spent catalyst standpipe 36. Anotherportion of the stripped coked catalyst may be recycled to the riser 20by the recirculation catalyst standpipe 19 without undergoingregeneration.

FIG. 1 depicts a regenerator 14 known as a combustor. However, othertypes of regenerators are suitable. In the catalyst regenerator 14, astream of oxygen-containing gas, such as air, is introduced through anair distributor 38 to contact the coked catalyst. Coke is combusted fromthe coked catalyst in a combustion chamber 80 to provide regeneratedcatalyst and flue gas. The catalyst regeneration process adds asubstantial amount of heat to the catalyst, providing energy to offsetthe endothermic cracking reactions occurring in the first riser 20.Catalyst and air flow upwardly together in the combustion chamber 80 ofregenerator 14 and, after regeneration, are initially separated bydischarge through a disengager 40 and enter a separation chamber 86.Additional recovery of the regenerated catalyst and flue gas exiting thedisengager 40 is achieved using first and second stage separatorcyclones 44, 46, respectively within the separation chamber 86 of thecatalyst regenerator 14. Catalyst separated from flue gas dispensesthrough diplegs from cyclones 44, 46 while flue gas relatively lighterin catalyst sequentially exits cyclones 44, 46 and exits the regeneratorvessel 14 through flue gas outlet 47 in a flue gas line 48. Regeneratedcatalyst is carried back to the riser 20 through the regeneratedcatalyst standpipe 18. As a result of the coke burning, the flue gasvapors exiting at the top of the catalyst regenerator 14 contain CO,CO2, N2 and H2O, along with smaller amounts of other species.

The product recovery section 8 is in downstream communication with theproduct outlet 31. In the product recovery section 8, the first crackedproduct stream in line 32 is directed to a lower section of an FCC mainfractionation column 92. The main column 92 is in downstreamcommunication with the first product outlet 31. Several fractions of FCCproduct may be separated and taken from the main column including aheavy slurry oil from the bottoms in line 93, a heavy cycle oil streamin line 94, a light cycle oil in line 95 taken from outlet 95 a and aheavy naphtha stream in line 96 taken from outlet 96 a. Any or all oflines 93-96 may be cooled and pumped back to the main column 92 to coolthe main column typically at a higher location. Gasoline and gaseouslight hydrocarbons are removed in a main overhead line 97 from the maincolumn 92 and condensed before entering a main column receiver 99. Themain column receiver 99 is in downstream communication with the productoutlet 31, and the main column 92 is in upstream communication with themain column receiver 99. The second hydrocarbon stream and perhaps athird hydrocarbon stream is taken from the main overhead line 97.

An aqueous stream is removed from a boot in the main column receiver 99.Moreover, a condensed light naphtha stream is removed in line 101 whilean overhead stream is removed in line 102. The overhead stream in line102 contains gaseous light hydrocarbon which is very olefinic. Thestreams in lines 101 and 102 may enter a vapor recovery section 120 ofthe product recovery section 8.

The vapor recovery section 120 is shown to be an absorption-basedsystem, but any vapor recovery system may be used including a cold boxsystem. To obtain sufficient separation of light gas components, thegaseous stream in line 102 is compressed in compressor 104. More thanone compressor stage may be used, but typically a dual stage compressionis utilized. The compressed light hydrocarbon stream in line 106 isjoined by streams in lines 107, 108 and 422, chilled and delivered to ahigh-pressure receiver 110. An aqueous stream from the receiver 110 maybe routed to the main column receiver 99. A gaseous hydrocarbon streamin line 112 is routed to a primary absorber 114 in which it is contactedwith unstabilized gasoline from the main column receiver 99 in line 101to effect a separation between C3+ and C2−hydrocarbons. The primaryabsorber 114 is in downstream communication with the main columnreceiver 99. A liquid C3+ hydrocarbon stream in line 107 is returned toline 106 prior to chilling. A primary off-gas stream in line 116 fromthe primary absorber 114 may be directed to a secondary absorber 118,where a circulating stream of light cycle oil in line 121 diverted fromline 95 absorbs most of the remaining C5+ and some C3-C4 hydrocarbons inthe primary off-gas stream. The secondary absorber 118 is in downstreamcommunication with the primary absorber 114. Light cycle oil from thebottom of the secondary absorber in line 119 richer in C3+ hydrocarbonsis returned to the main column 92 via the pump-around for line 95. Theoverhead of the secondary absorber 118 comprising dry gas ofpredominantly C2-hydrocarbons with hydrogen sulfide, ammonia, carbonoxides and hydrogen is removed in a secondary off-gas stream in line122.

Liquid from the high-pressure receiver 110 in line 124 is sent to astripper 126. Most of the C2− hydrocarbons is removed in the overhead ofthe stripper 126 and returned to line 106 via overhead line 108. Aliquid bottoms stream from the stripper 126 is sent to a firstdebutanizer column 130 in a bottoms line 128. The first debutanizercolumn 130 provides an overhead stream in line 132 comprising a C3-C4hydrocarbon stream from the first debutanizer column. A bottoms streamin line 134 may comprise a first debutanized naphtha stream.

In a first embodiment, a first recycle light cracked naphtha stream maybe taken in line 137 through a control valve thereon from the firstdebutanized naphtha stream in line 134 while the remainder of the firstdebutanized naphtha stream in line 141 may be further processed intogasoline or other products through a control valve thereon. In thisembodiment it is envisioned that a naphtha splitter column 136 may belocated upstream in the product recovery section 8.

In an alternative embodiment, the first debutanized naphtha stream inline 134 may be fed to the naphtha splitter column 136 in line 142through a control valve thereon. In this alternative embodiment, thenaphtha splitter column is located downstream in the product recoverysection 8 as depicted in the FIGURE and the control valves on lines 137and 141 will be closed. The naphtha splitter column separates thedebutanized naphtha stream into a first split light naphtha stream in anoverhead line 138 comprising C5-C7 hydrocarbons and a heavy naphthastream in a bottoms line 140. An alternative first recycle light crackednaphtha stream may be taken in line 139 through a control valve thereonwhile the remainder of the first split light naphtha stream in theoverhead line 138 may be further processed into gasoline or otherproducts.

The C3-C4 hydrocarbon stream taken in line 132 may be separated in aC3-C4 splitter column 144 into a C3 hydrocarbon stream in an overheadline 146 and a C4 hydrocarbon stream in a bottoms line 148. A firstrecycle C4 hydrocarbon stream may be taken in line 149 through a controlvalve thereon while the remainder C4 hydrocarbon stream may be furtherprocessed into other products in line 147. The C3 hydrocarbon stream inthe overhead line 146 may be further processed for propylene recovery.

One or both of the first recycle light cracked naphtha stream taken inline 137 from the first debutanized naphtha stream in line 134 from thefirst debutanizer column 130 in downstream communication with the maincolumn 92 comprising olefinic C5-C7 hydrocarbons or the alternativefirst recycle light cracked naphtha stream taken from an overhead line138 of the naphtha splitter column 136 in downstream communication withthe main column 92 in line 139 comprising olefinic C5-C7 hydrocarbonsand the first recycle C4 stream comprising olefinic C4 hydrocarbonstaken from a bottoms line 148 of the C3-C4 splitter column also indownstream communication with the main column 92 in line 149 may berecycled to a second FCC reactor 202 in a second charge line 150 as thesecond hydrocarbon stream. The second hydrocarbon stream may bepreheated to a temperature of about 221° C. (400° F.) to about 704° C.(1300° F.) and charged to the second FCC reactor 202.

The FIGURE shows a second FCC reactor 200 comprising a second reactorunit 202 and a catalyst heater 238. The second reactor unit 202 includesa second riser 212 in which the second hydrocarbon stream in line 150charged through a distributor 213 or more near the base of the secondriser 212 is contacted with a second stream of fluid catalyst in asecond riser 212. The second hydrocarbon stream may comprise at least 20wt % olefins, suitably at least wt % olefins and preferably at least 70wt % olefins. The second hydrocarbon stream may comprise at least 1 wt %paraffins, suitably at least 15 wt % paraffins and preferably at least25 wt % paraffins. The second hydrocarbon stream may comprise oncecracked C4 to C7 hydrocarbons.

The second riser 212 extends upwardly through a second reactor vessel210 in the second FCC reactor 202. A second stream of fluid catalyst maybe fluidized with steam distributed from a distributor 218 at a bottomof the second riser 212. The second hydrocarbon stream is contacted witha second stream of fluid catalyst in the second riser 212. The secondstream of fluid catalyst may be provided by a mixture of a first streamof hot catalyst from a first hot catalyst pipe 220 and a first stream ofrecycle catalyst from a first recycle catalyst pipe 222. The secondhydrocarbon stream vaporizes and converts or cracks to a second crackedproduct stream comprising ethylene and propylene in greaterconcentration than in the second hydrocarbon stream. Molar expansioncauses the second hydrocarbon stream and the second cracked productstream to rapidly ascend the second riser 212 entraining the secondstream of fluid catalyst as a second mixture of catalyst and productgas.

The second stream of fluid catalyst can comprise less than about 20 wt%, preferably less than about 5 wt %, of the first component and atleast 20% by weight, of the second component. In one preferredembodiment, the second stream of fluid catalyst can include at leastabout 20 wt % of a ZSM-5 zeolite and less than about 20 wt %, preferablyless than about 5 wt % of a Y-zeolite. In another preferred embodiment,the second stream of fluid catalyst can predominantly comprise thesecond component and in a further embodiment can contain only the secondcomponent, preferably a ZSM-5 zeolite, as the catalyst.

In an aspect, the second stream of fluid catalyst may comprise coke fromabout 0.005 wt % to about 1.2 wt % coke. The presence of coke in thisconcentration passivates acid sites to help in preventing the productionof dry gas. The dry gas may be defined as H₂, H₂S, carbon oxides, andC₁-C₂ hydrocarbons. Dry gas represents a loss in yield of valuableproducts and increases operation costs and equipment costs resultingfrom the handling of greater gas flow rates. The presence of coke in anamount from about 0.005 wt % to about 1.2 wt % in the riser sufficientlypassivates acid site which in turn limits the production of dry gas.

Coke concentration is increased in the second riser by circulatingunregenerated catalyst to the riser and promoting coke generation in theriser. If the FCC reaction generates coke on the catalyst recycle of afirst stream of catalyst in a first recycle catalyst pipe 222 to thesecond riser 212 will increase coke in the riser because it bypassesregeneration. Increasing the recycle rate of the first recycle catalystrelative to the recycle rate of the first stream of hot catalyst from afirst hot catalyst pipe 220 will increase coke concentration in thesecond riser. Some types of catalyst, such as ZSM5, generate little cokein an FCC reactor. If coke generation in the second riser is notsufficient, other ways may be used to increase coke concentration in thesecond riser which will be described hereinafter.

Process conditions in the second riser 212 will be more severe than inthe first riser 20 because the second hydrocarbon stream is lesscrackable than the first hydrocarbons stream due to the former havingbeen previously cracked in the first riser 20. The second riser 212 mayoperate at one or more of the following conditions relative to the firstriser 20: a higher outlet temperature, a lower hydrocarbon partialpressure or a different catalyst density. Hydrocarbon partial pressureis reduced by reducing the total pressure in the second riser 212independent of the pressure in the first FCC reactor 10 and perhapsadjusting the steam rate to the second riser 212.

Conditions in the second riser 212 may include a cracking reactiontemperature of 400° to 650° C., preferably about 565° C. to about 635°C. at the reactor outlet. The cracking occurs at an absolute pressurebetween about 100 kPa (14 psia) to about 506 kPa (74 psia), preferablybetween about 138 kPa (20 psia) to about 310 kPa (45 psia). A steam flowrate of about 5 to about 25 wt % of the second hydrocarbon stream isadded to the second riser 20. However, the steam rate in the secondriser 212 can be as low as about 2 to about 25 wt % or it can beeliminated. Control valves on the first hot catalyst pipe 220 and on thefirst recycle catalyst pipe 222 can be used to adjust the catalystdensity in the second riser 212 thus enabling control of the spacevelocity therein. Also, increasing the flow rate of the first recyclecatalyst to the second riser 212 in the first recycle catalyst pipe 222can increase the catalyst density in the second riser without impactingthe heat input to the second riser supplied the first stream of hotcatalyst in the first hot catalyst pipe 220 that is fed to the secondriser 212.

The second riser 212 terminates in an upper end of a seconddisengagement chamber 211 located within the second reactor vessel 210at a curved duct 214 or a plurality thereof. The curved duct 214 maycentrifugally discharge a second mixture of product gas and catalystinto the second disengagement chamber 211. By centrifugal discharge, thefirst mixture is discharged from inwardly to outwardly. Centrifugaldischarge of gases and catalyst produces a swirling helical patternabout the interior of the second disengagement chamber 211 to effect adisengagement of the second mixture of catalyst and product gas into asecond cracked product stream and a first stream of cool catalyst in thesecond disengagement chamber 211.

The first stream of cool catalyst collects in a dense catalyst bed 228.The second stream of product gas passes upwardly through a second gasrecovery conduit 226, is further separated from catalyst in cyclones 232and is discharged from the second reactor vessel 210 through an outlet230 in product line 231.

The FIGURE illustrates a third FCC reactor unit 302 in which a thirdhydrocarbon stream in line 315 distributed through a distributor 313 ormore near the base of a third riser 312 is contacted with a third streamof fluid catalyst in the third riser. In an embodiment, the third FCCreactor unit 302 is integrated in the second FCC reactor 200 with thesecond FCC reactor unit 202. However, the third FCC reactor unit 302 maystand alone from the second FCC reactor unit 202 in its own FCC reactor.The third hydrocarbon stream may comprise at least 20 wt % olefins,typically at least 30 wt % olefins, suitably at least 50 wt % olefinsand preferably at least 60 wt % olefins. The third hydrocarbon streammay comprise at least 25 wt % paraffins and preferably at least 35 wt %paraffins. The second hydrocarbon stream is typically more olefinicand/or more crackable than the third hydrocarbon stream. The thirdhydrocarbon stream may comprise a twice cracked light cracked naphthastream and/or a twice cracked C4 hydrocarbon stream and/or a oncecracked C4 hydrocarbon stream. The third hydrocarbon stream may bepreheated to a temperature of about 221° C. (400° F.) to about 704° C.(1300° F.) and charged to the third FCC reactor 302.

The third stream of catalyst may be fluidized with steam distributedfrom a distributor 358 at a bottom of the third riser 312. The thirdstream of catalyst may have the same catalyst composition as the secondstream of catalyst. The first stream of catalyst has a catalystcomposition that is different from the catalyst composition of thesecond stream of catalyst and the third stream of catalyst. The thirdhydrocarbon stream is contacted with the third stream of fluid catalystin the third riser 312. The third stream of fluid catalyst may beprovided by a mixture of a second stream of hot catalyst from a secondhot catalyst pipe 362 and a second stream of recycle catalyst from asecond recycle catalyst pipe 264. The third hydrocarbon feedstockconverts or cracks to a third cracked product stream comprisinghydrocarbons of smaller molecular weight than the third hydrocarbonstream. Molar expansion causes the third hydrocarbon stream and thethird cracked product stream to rapidly ascend the third riser 312entraining the third stream of fluid catalyst as a third mixture ofcatalyst and product gas.

Process conditions in the third riser 312 may be more severe than in thesecond riser 212 and the first riser 20. The third riser 312 may operateat one or more of the following conditions relative to the second riser212: a higher outlet temperature, a lower hydrocarbon partial pressureand a different catalyst density than the second riser. Hydrocarbonpartial pressure may be reduced by reducing total pressure in the thirdriser 312 independent of the pressure in the first FCC reactor 10 andperhaps adjusting the steam rate to the third riser 312.

Conditions in the third riser 312 may include a cracking reactiontemperature of about 400° C. to about 705° C., preferably about 565° C.to about 675° C. at the reactor outlet. The cracking occurs at anabsolute pressure between about 100 kPa (14 psia) to about 506 kPa (74psia), preferably between about 138 kPa (20 psia) to about 310 kPa (45psia). Steam of about 25 to about 50 wt % of third hydrocarbon streamrate is added to the third riser 312. However, the steam rate in thethird riser 312 can be as low as about 2 to about 50 wt % and it can beeliminated. Control valves on the second hot catalyst pipe 362 and onthe second recycle catalyst pipe 264 can be used to adjust the catalystdensity in the third riser 312 thus enabling control of the spacevelocity therein. Also, increasing the flow rate of the spent, secondrecycle catalyst stream to the third riser 312 in the second recyclecatalyst pipe 264 can increase the catalyst density in the third riser312 without impacting heat input to the third riser.

Coke concentration is increased in the third riser 312 by circulatingunregenerated catalyst to the riser and promoting coke generation in theriser. If the FCC reaction generates coke on the catalyst recycle of asecond stream of recycle catalyst in a second recycle catalyst pipe 264to the third riser 312 will increase coke in the riser because itbypasses regeneration. Increasing the recycle rate of the second recyclecatalyst stream relative to the recycle rate of the second stream of hotcatalyst from a second hot catalyst pipe 362 will increase cokeconcentration in the third riser 312. Some types of catalyst, such asZSM5, generate little coke in an FCC reactor. If coke generation in thethird riser 312 is not sufficient, other ways may be used to increasecoke concentration in the third riser which will be described hereinafter.

The third riser 312 of the third FCC reactor 302 may be located externalto the second reactor vessel 210 but share the second reactor vesselwith the second riser 212 and the second FCC reactor 202. The thirdriser 312 comprises a discharge opening 349 in a third disengagementchamber 360. The third disengagement chamber 360 contains the dischargeopening 349 of the third riser 312. The third disengagement chamber 360may be in the second reactor vessel 210. In an embodiment, a horizontaltransfer line 348 of the third riser 312 terminates in the thirddisengagement chamber 360. The discharge opening 349 of the third riser302 tangentially discharges the third mixture of catalyst and productgas into the third disengagement chamber 360. In other embodiments, thehorizontal transfer line 348 may be exchanged for an alternativeconnector such a T-type connector or an elbow with a more acute or moreobtuse angle. Tangential discharge of the third mixture of catalyst andproduct gas through the discharge opening 349 from the third riser 312produces a swirling helical pattern about the interior of the thirddisengagement chamber 360. The disengagement of the third mixture ofcatalyst and product gas into a second stream of cool catalyst and athird cracked product stream may be conducted outwardly andconcentrically of the disengagement of the second mixture of catalystand product gas into a second cracked product stream and the firststream of cool catalyst. It is important that the second mixture ofcatalyst and product gas does not mix with the third mixture of catalystand product gas until the bulk of the catalyst is removed from theproduct gas to maximize selectivity to propylene.

In an embodiment, the second stream of cool catalyst collects in thedense catalyst bed 228 along with the first stream of cool catalyst. Ina further embodiment, the third stream of cracked product passesupwardly through the second gas recovery conduit 226 along with thesecond stream of cracked product, is further separated from catalyst incyclones 232 and is discharged from the second reactor vessel 210through an outlet 230 in the product line 231 as a second productstream.

A mixed stream of disengaged cool catalyst from the dense catalyst bed228 passes downwardly through a stripping section 284. A strippingfluid, typically steam enters a lower portion of stripping section 284through a distributor 234. Countercurrent contact of the catalyst withthe stripping fluid through a series of stripping baffles, packing orgrates displaces product gases from the catalyst as it continuesdownwardly through the stripping section 284.

A first stream of stripped catalyst from the stripping section 284passes through a heater conduit 236 to a catalyst heater 238. In oneembodiment, the catalyst heater 238 heats the catalyst by heat exchangewith regenerated catalyst from the regenerator. In the secondembodiment, the catalyst heater 238 contacts the first stream ofstripped catalyst with an oxygen supply gas from line 219 to combustcoke from the catalyst. Flue gas is discharged in line 242. The secondembodiment is applicable when the catalyst generates ample coke in theFCC reaction. The catalyst heater 238 provides the first stream of hotcatalyst in the first hot catalyst pipe 220 that is fed to the secondriser 212 and the second stream of hot catalyst in the second hotcatalyst pipe 362 that is fed to the third riser 312.

A second stream of stripped catalyst from the dense bed 228 passes in arecycle conduit 240 to provide the first stream of recycle catalyst inthe first recycle catalyst pipe 222 to the second riser 212 and thesecond stream of recycle catalyst in the second recycle catalyst pipe264 to the third riser 312. In the first embodiment of the catalystheater 238, the catalyst in the second FCC reactor 202 and the third FCCreactor 302 may not coke up as much as in the first FCC reactor 12.Hence, in the second and third FCC reactors 202, 302, insufficient cokemay be burned to balance heat demands in the reactors. To supplementheat to the second FCC reactor 202 and the third FCC reactor 302, aportion of the hot regenerated catalyst may be transported to a catalystheater 238 in a regenerator heater standpipe 42. In the catalyst heater,a portion of the second stream of fluid catalyst may be heat exchangedwith a portion of the hot regenerated catalyst before contacting thesecond stream of fluid catalyst with the second hydrocarbon stream.Moreover, a portion of the third stream of fluid catalyst may be heatexchanged with a portion of the hot regenerated catalyst beforecontacting the third stream of fluid catalyst with the third hydrocarbonstream. In the catalyst heater 238 of the first embodiment hotregenerated catalyst may be on one side of an indirect heat exchangerwhile the second stream of fluid catalyst and the third stream of fluidcatalyst may be on the other side of the indirect heat exchanger.Catalyst from the second FCC reactor 202 and third FCC reactor 302 maybe delivered to the catalyst heater 238 by the heater conduit 236.Cooled regenerated catalyst can be returned to the regenerator 14 fromthe catalyst heater 238 in a cooled regenerated catalyst conduit 48.

It is envisioned that a heat exchange fluid may also be used in thefirst embodiment of the catalyst heater 238 to transfer heat to thesecond and third catalyst streams from the regenerated catalyst insteadof by direct heat exchange between catalyst streams. In an embodiment,heat can be exchanged either by direct or indirect means from flue gasfrom flue gas outlet 47 to heat the second and third catalyst streams.In another embodiment, the catalyst heater 238 may have fuel firing tosupply the heat needed. A portion of the second stream of fluid catalystgets heated in the catalyst heater 238 before contacting said secondstream of fluid catalyst with the second hydrocarbon stream. A portionof the third stream of fluid catalyst gets heated in the catalyst heater238 before contacting said third stream of fluid catalyst with the thirdhydrocarbon stream. A flue gas stream in line 242 generated out of thecatalyst heater 238 may be appropriately routed to the flue gastreatment unit. In another embodiment, the fuel firing in catalystheater 238 may be replaced by electrical coils powered by renewable orfossil fuel-based electricity.

To generate more coke on catalyst, a C4+ hydrocarbon stream may be fedto the catalyst heater 238 of the first embodiment in a first cokingline 221 directly to catalyst in the catalyst heater 238 or one or moreof a second coking line 223 to the first stream of hot catalyst in thefirst hot catalyst pipe 220, a third coking line 225 to the secondstream of hot catalyst in the second hot catalyst pipe 362, a fourthcoking line 227 to the recycle catalyst in the first recycle catalystpipe 222, and a fifth coking line 229 to the recycle catalyst in thesecond recycle catalyst pipe 264. The C4+ injection in the catalystpipes will help optimize the coke level in the catalyst which maximizespropylene selectivity in second and third risers. C4+ hydrocarboninjections should be located downstream of the control valve in thecatalyst pipes. The coke distribution in the catalyst shall be about0.005 to about 5 wt % coke on catalyst, preferentially about 0.1 toabout 2 wt % coke on catalyst in the second riser 212, and about 0.1 toabout 1 wt % coke on catalyst in the third riser 312. In an embodiment,the coke on catalyst may be about 0.005 to about 1.2 wt % coke oncatalyst in the second riser 212 and in the third riser 312. Ahydrocarbon stream may also be fed to the catalyst heater 238 of thesecond embodiment in the first coking line 221 if necessary to increasethe heat of combustion to provide more heat input for the FCC reactionin the second reactor 202 and third reactor 302.

The coked catalyst from stripping section 284 can be dispensed directlyand periodically to the catalyst regenerator 14 in a dispense conduit250. The dispensed catalyst can combine with the catalyst mixture in thecatalyst regenerator 14 and provide additional catalyst activity to thecatalyst in the FCC unit section 6.

The second product stream in line 231 from the second riser 212 and thethird riser 312 may be passed to a wash column 392. In the wash column392, the second product stream in line 231 is contacted with a washstream. In an embodiment, the wash stream may be a fresh charge streamin line 394 that is contacted with the hot second product stream toeffect a direct heat exchange. The direct heat exchange quenches the hotsecond product stream and absorbs catalyst fines from the second productstream. The quenched charge stream with catalyst fines is dischargedfrom the bottom of the wash column 392 in line 15 and taken as the firsthydrocarbon stream charged to the first FCC reactor 12.

In another embodiment, the second product stream in line 231 may be usedfor preheating the second hydrocarbon stream 150 and/or thirdhydrocarbon stream 315 by indirect heat exchange before it is passed tothe wash column 392. In another embodiment, the second hydrocarbonstream 150 and/or third hydrocarbon stream 315 may be preheated by theflue gas stream 48 from flue gas outlet 47.

The quenched second product overhead stream is taken from a washoverhead line 396 from a top of the wash column 392 after some stages ofcooling pump-arounds to further cool the second product stream in line396 to the product recovery section 90. The wash column 392 is indownstream communication with the second product outlet 231. In theproduct recovery section 90, the second product stream in the washoverhead line 396 is directed to a wash column receiver 399. The washcolumn receiver 399 is in downstream communication with the secondproduct outlet 231.

An aqueous stream is removed from a boot in the wash column receiver399. Moreover, a condensed light naphtha stream is removed in line 401while an overhead gas stream is removed in wash receiver overhead line402. The overhead stream in line 402 contains gaseous light hydrocarbonwhich are very olefinic. The streams in lines 401 and 402 may enter awash recovery section 420 of the product recovery section 90. In anotherembodiment, the wash column 392 may be an integral part of productrecovery section 8 by making use of a downstream depropanizer columntherein that is not shown and the first debutanizer column 130 in thevapor recovery section 120.

The wash recovery section 420 is shown to be an absorption-based system,but any vapor recovery system may be used including a cold box system.To obtain sufficient separation of light gas components, the overheadstream in line 402 is compressed in compressor 404. More than onecompressor stage may be used, but typically a dual stage compression isutilized. A compressed light hydrocarbon stream in line 406 is deliveredto a high-pressure receiver 410. Aqueous streams may be taken from thereceivers 399 and 410. A high-pressure liquid stream from a bottom ofthe receiver 410 in line 412 is routed to a depropanizer column 420 withthe light naphtha stream in line 401 from a bottom of the wash columnreceiver 399 in line 414.

The depropanizer column 420 separates the high-pressure liquid stream inline 412 and the light naphtha stream in line 401 into a C3−hydrocarbonstream in line 422 and adds it to the compressed light hydrocarbonstream in line 106 to be processed therein. A depropanized bottom streamin line 424 may be routed to a debutanizer flash drum 426. In thedebutanizer flash drum 426 the depropanized bottom stream is separatedinto a second C4 hydrocarbon stream in an overhead line 432 and adebutanizer naphtha stream in the bottoms line 434. The second C4hydrocarbon stream may be recycled in line 432 through a control valvethereon to the second recycle stream in line 314.

The debutanizer feed stream in line 434 is fed to a second debutanizercolumn 440. The second debutanizer column 440 separates the debutanizerfeed stream into a C4 product stream in an overhead line 442 and asecond light cracked naphtha stream in the bottoms line 444. A secondrecycle light cracked naphtha stream may be taken in line 446 through acontrol valve thereon while the remainder may be further processed intogasoline or other products. The second recycle light cracked naphthastream in line 446 may be fed to the second recycle stream in the secondrecycle line 314.

One or both of the second recycle light cracked naphtha stream takenfrom the second debutanizer column 440 in downstream communication withthe wash column 392 in line 446 comprising olefinic C5-C7 hydrocarbonsand the second recycle C4 stream comprising olefinic C4 hydrocarbonstaken from the debutanizer flash drum 426 also in downstreamcommunication with the wash column 392 in line 432 may be recycled tothe third FCC reactor 302 as the third hydrocarbon stream in a thirdcharge line 315. The third hydrocarbon stream may be preheated to atemperature of about 204° C. (400° F.) to about 704° C. (1300° F.) andcharged to the third FCC reactor 302.

The second hydrocarbon stream and/or the third hydrocarbon stream can besupplemented with C4 hydrocarbon stream, a C5 hydrocarbon stream or alight cracked naphtha stream from a refinery or a steam cracking unit.Such streams will typically comprise more than 20 wt % olefins.

In a further embodiment, some or all of the first C4 hydrocarbon streamin line 149 may be taken in line 160 through a control valve thereon andfed to the second recycle line 314. The first C4 hydrocarbon stream fedto the second recycle line 314 may be taken in the third charge line 315as the third hydrocarbon stream as all or part of the third hydrocarbonstream.

The third FCC reactor 302 can be used to further crack olefinic feeds toproduce additional propylene by cracking a less crackable feed in athird riser 312 at conditions that are more severe than in the secondriser 212 and which are uniquely favorable to propylene production forthe third hydrocarbon stream.

EXAMPLES Example 1

A hydrocarbon feed was cracked in three separate risers having thecompositions in the Table. C4 hydrocarbons from the first riser effluentwas charged to the second riser, and consequentially C4 hydrocarbonsfrom the second riser were fed to the third riser. Only propylene wasreported in the third riser effluent. Yields are provided in Table 1.Riser conditions are in line with what is taught in the DetailedDescription.

TABLE 1 1^(st) Riser 2^(nd) Riser 3^(rd) Riser Feed VGO, wt % 100.0% — —C4 Paraffin, wt % — 21.9% 39.8% C4 Olefin, wt % — 78.1% 60.2% ProductsPropylene, wt % 17.5% 28.2% 43.1% Propane, wt % 1.6% 2.0% UnreportedButylenes, wt % 13.5% 33.9% Unreported i-Butane, wt % 2.8% 15.7%Unreported n-Butane, wt % 1.0% 6.7% Unreported Propylene, wt % of VGO17.5% 4.9% 4.2%By use of three separate risers, over 26 wt % of propylene as apercentage of feed can be produced.

Example 2

A hydrocarbon feed was cracked in two separate risers having thecompositions in the Table. All C5-C7 hydrocarbons from the first risereffluent was fed to the second riser. In case 1, the MFI-type catalystwas fully regenerated in the second riser, so the catalyst contains lessthan 0.005 wt % coke. In case 2, the catalyst in the second risercontained coke with distribution in the range of 0.005 to 1.2 wt % cokeon the MFI-type catalyst. The total product yields of the first andsecond risers are provided in Table 2. Riser conditions are in line withwhat is taught in the Detailed Description.

TABLE 2 1^(st) + 2^(nd) Risers 1^(st) + 2^(nd) Risers 1^(st) Riser Case1 Case 2 Feed Resid, wt % 100.0% Products Dry Gas, wt % Resid 3.36%6.34% 4.68% Propylene, wt % Resid 10.04% 14.37% 15.52% Propane, wt %Resid 1.84% 2.13% 2.26% Butylenes, wt % Resid 9.13% 10.94% 11.33%i-Butane, wt % Resid 3.2% 3.48% 3.63% n-Butane, wt % Resid 0.9% 1.73%1.89% Gasoline, wt % Resid 35.54% 24.84% 25.07% LCO, wt % Resid 20.38%20.48% 20.23% CSO, wt % Resid 7.38% 7.42% 7.30% Coke, wt % Resid 8.07%8.11% 8.09%By recycling the C5-C7 hydrocarbons from the first riser to the secondriser, more light olefins were produced, such as ethylene, propylene,and butylene. On ZSM-5 catalyst, the propylene selectivity of 40-60% isachieved. Partially coked catalyst enhances the propylene and butyleneyields while reducing the dry gas and heavies production.

Specific Embodiments

While the following is described in conjunction with specificembodiments, it will be understood that this description is intended toillustrate and not limit the scope of the preceding description and theappended claims.

A first embodiment of the invention is a process for catalyticproduction of olefins comprising contacting a first hydrocarbon streamand a first stream of fluid catalyst in a first riser to produce a firstcracked product stream and a spent catalyst stream; separating the firstcracked product stream in a main column; separating an overhead streamfrom the main column into a second hydrocarbon stream; contacting thesecond hydrocarbon stream with a second stream of fluid catalyst in asecond riser to produce a second cracked product stream and a firststream of cool catalyst; and obtaining a third hydrocarbon stream fromthe overhead stream and/or from the second cracked product stream; andcontacting the third hydrocarbon stream with a third stream of fluidcatalyst in a third riser to produce a third cracked product stream anda second stream of cool catalyst. An embodiment of the invention is one,any or all of prior embodiments in this paragraph up through the firstembodiment in this paragraph wherein the second hydrocarbon stream ismore olefinic and/or more crackable than the third hydrocarbon stream.An embodiment of the invention is one, any or all of prior embodimentsin this paragraph up through the first embodiment in this paragraphwherein the second hydrocarbon stream is a light cracked naphtha streamand further comprising separating the overhead stream from the maincolumn into a first C4 hydrocarbon stream and taking the first C4hydrocarbon stream as the third hydrocarbon stream. An embodiment of theinvention is one, any or all of prior embodiments in this paragraph upthrough the first embodiment in this paragraph further comprisingwashing the second cracked product stream in a wash column and obtainingthe third hydrocarbon stream from the wash column. An embodiment of theinvention is one, any or all of prior embodiments in this paragraph upthrough the first embodiment in this paragraph wherein the secondhydrocarbon stream is a first C4 hydrocarbon stream and the thirdhydrocarbon stream is a second C4 hydrocarbon stream. An embodiment ofthe invention is one, any or all of prior embodiments in this paragraphup through the first embodiment in this paragraph further comprisingcompressing an overhead stream from the wash column to provide acompressed overhead stream and depropanizing the compressed overheadstream to provide a depropanized compressed overhead stream and takingthe third hydrocarbon stream from the depropanized compressed overheadstream. An embodiment of the invention is one, any or all of priorembodiments in this paragraph up through the first embodiment in thisparagraph further comprising separating a depropanized compressedoverhead stream into a second C4 stream and a second light crackednaphtha stream and taking the third hydrocarbon stream from the secondlight cracked naphtha stream and/or from the second C4 stream. Anembodiment of the invention is one, any or all of prior embodiments inthis paragraph up through the first embodiment in this paragraph whereinthe second stream of fluid catalyst and the third stream of fluidcatalyst comprise about 0.005 to about 1.2 wt % coke. An embodiment ofthe invention is one, any or all of prior embodiments in this paragraphup through the first embodiment in this paragraph wherein the thirdriser operates at a higher outlet temperature and/or a lower hydrocarbonpartial pressure and/or a different catalyst density than the secondriser. An embodiment of the invention is one, any or all of priorembodiments in this paragraph up through the first embodiment in thisparagraph further comprising regenerating the stream of spent catalystby combustion of coke from the spent catalyst to provide hot regeneratedcatalyst and heat exchanging a portion of the second stream of fluidcatalyst with a portion of the hot regenerated catalyst beforecontacting the second stream of fluid catalyst with the secondhydrocarbon stream. An embodiment of the invention is one, any or all ofprior embodiments in this paragraph up through the first embodiment inthis paragraph further comprising regenerating the stream of spentcatalyst by combustion of coke from the spent catalyst to provide hotregenerated catalyst and heat exchanging a portion of the third streamof fluid catalyst with a portion of the hot regenerated catalyst streambefore contacting the third stream of fluid catalyst with the thirdhydrocarbon stream. An embodiment of the invention is one, any or all ofprior embodiments in this paragraph up through the first embodiment inthis paragraph wherein the third hydrocarbon stream has more than 20 wt% olefins. An embodiment of the invention is one, any or all of priorembodiments in this paragraph up through the first embodiment in thisparagraph further comprising recycling a C4+ stream to a catalyst pipethat provides catalyst to the second riser and/or to the third riser.

A second embodiment of the invention is a process for catalyticproduction of olefins comprising contacting a first hydrocarbon streamand a first stream of fluid catalyst in a first riser to produce a firstcracked product stream and a spent catalyst stream; separating the firstcracked product stream in a main column; obtaining a second hydrocarbonstream from the main column; contacting the second hydrocarbon streamwith a second stream of fluid catalyst in a second riser to produce asecond cracked product stream and a first stream of cool catalyst; andseparating the second cracked product stream in a wash column andobtaining the third hydrocarbon stream from the wash column; andcontacting the third hydrocarbon stream with a third stream of fluidcatalyst in a third riser to produce a third cracked product stream anda second stream of cool catalyst. An embodiment of the invention is one,any or all of prior embodiments in this paragraph up through the secondembodiment in this paragraph further comprising compressing an overheadstream from the wash column to provide a compressed overhead stream anddepropanizing the compressed overhead stream to provide a depropanizedcompressed overhead stream and taking the third hydrocarbon stream fromthe depropanized compressed overhead stream. An embodiment of theinvention is one, any or all of prior embodiments in this paragraph upthrough the second embodiment in this paragraph wherein the thirdhydrocarbon stream has more than 20 wt % olefins.

A third embodiment of the invention is a process for catalyticproduction of olefins comprising contacting a first hydrocarbon streamand a first stream of fluid catalyst in a first riser to produce a firstcracked product stream and a spent catalyst stream; separating the firstcracked product stream in a main column; obtaining a second hydrocarbonstream from an overhead line of the main column system; contacting thesecond hydrocarbon stream with a second stream of fluid catalyst in asecond riser to produce a second cracked product stream and a firststream of cool catalyst; and obtaining a third hydrocarbon stream fromthe first cracked product stream or the second cracked product stream;and contacting the third hydrocarbon stream with a third stream of fluidcatalyst in a third riser to produce a third cracked product stream anda second stream of cool catalyst. An embodiment of the invention is one,any or all of prior embodiments in this paragraph up through the thirdembodiment in this paragraph wherein the second hydrocarbon stream ismore olefinic and/or more crackable than the third hydrocarbon stream.An embodiment of the invention is one, any or all of prior embodimentsin this paragraph up through the third embodiment in this paragraphwherein the third hydrocarbon stream has more than 20 wt % olefins. Anembodiment of the invention is one, any or all of prior embodiments inthis paragraph up through the third embodiment in this paragraph furthercomprising operating the third riser at a greater severity than thesecond riser.

Without further elaboration, it is believed that using the precedingdescription that one skilled in the art can utilize the presentinvention to its fullest extent and easily ascertain the essentialcharacteristics of this invention, without departing from the spirit andscope thereof, to make various changes and modifications of theinvention and to adapt it to various usages and conditions. Thepreceding preferred specific embodiments are, therefore, to be construedas merely illustrative, and not limiting the remainder of the disclosurein any way whatsoever, and that it is intended to cover variousmodifications and equivalent arrangements included within the scope ofthe appended claims.

In the foregoing, all temperatures are set forth in degrees Celsius and,all parts and percentages are by weight, unless otherwise indicated.

1. A process for catalytic production of olefins comprising: contactinga first hydrocarbon stream and a first stream of fluid catalyst in afirst riser to produce a first cracked product stream and a spentcatalyst stream; separating said first cracked product stream in a maincolumn; separating an overhead stream from the main column into a secondhydrocarbon stream; contacting said second hydrocarbon stream with asecond stream of fluid catalyst in a second riser to produce a secondcracked product stream and a first stream of cool catalyst; andobtaining a third hydrocarbon stream from said overhead stream and/orfrom said second cracked product stream; and contacting said thirdhydrocarbon stream with a third stream of fluid catalyst in a thirdriser to produce a third cracked product stream and a second stream ofcool catalyst.
 2. The process of claim 1 wherein said second hydrocarbonstream is more olefinic and/or more crackable than said thirdhydrocarbon stream.
 3. The process of claim 1 wherein said secondhydrocarbon stream is a light cracked naphtha stream and furthercomprising separating said overhead stream from the main column into afirst C4 hydrocarbon stream and taking said first C4 hydrocarbon streamas said third hydrocarbon stream.
 4. The process of claim 1 furthercomprising washing said second cracked product stream in a wash columnand obtaining said third hydrocarbon stream from said wash column. 5.The process of claim 3 wherein said second hydrocarbon stream is a firstC4 hydrocarbon stream and said third hydrocarbon stream is a second C4hydrocarbon stream.
 6. The process of claim 1 further comprisingcompressing an overhead stream from said wash column to provide acompressed overhead stream and depropanizing said compressed overheadstream to provide a depropanized compressed overhead stream and takingsaid third hydrocarbon stream from said depropanized compressed overheadstream.
 7. The process of claim 6 further comprising separating adepropanized compressed overhead stream into a second C4 stream and asecond light cracked naphtha stream and taking said third hydrocarbonstream from said second light cracked naphtha stream and/or from saidsecond C4 stream.
 8. The process of claim 1 wherein said second streamof fluid catalyst and said third stream of fluid catalyst contain about0.005 to about 1.2 wt % coke.
 9. The process of claim 1 wherein saidthird riser operates at a higher outlet temperature and/or a lowerhydrocarbon partial pressure and/or a different catalyst density thansaid second riser.
 10. The process of claim 1 further comprisingregenerating said stream of spent catalyst by combustion of coke fromsaid spent catalyst to provide hot regenerated catalyst and heatexchanging a portion of said second stream of fluid catalyst with aportion of said hot regenerated catalyst before contacting said secondstream of fluid catalyst with said second hydrocarbon stream.
 11. Theprocess of claim 1 further comprising regenerating said stream of spentcatalyst by combustion of coke from said spent catalyst to provide hotregenerated catalyst and heat exchanging a portion of said third streamof fluid catalyst with a portion of said hot regenerated catalyst streambefore contacting said third stream of fluid catalyst with said thirdhydrocarbon stream.
 12. The process of claim 1 wherein the thirdhydrocarbon stream has more than 20 wt % olefins.
 13. The process ofclaim 1 further comprising recycling a C4+ stream to a catalyst pipethat provides catalyst to the second riser and/or to the third riser.14. A process for catalytic production of olefins comprising: contactinga first hydrocarbon stream and a first stream of fluid catalyst in afirst riser to produce a first cracked product stream and a spentcatalyst stream; separating said first cracked product stream in a maincolumn; obtaining a second hydrocarbon stream from said main column;contacting said second hydrocarbon stream with a second stream of fluidcatalyst in a second riser to produce a second cracked product streamand a first stream of cool catalyst; and separating said second crackedproduct stream in a wash column and obtaining said third hydrocarbonstream from said wash column; and contacting said third hydrocarbonstream with a third stream of fluid catalyst in a third riser to producea third cracked product stream and a second stream of cool catalyst. 15.The process of claim 14 further comprising compressing an overheadstream from said wash column to provide a compressed overhead stream anddepropanizing said compressed overhead stream to provide a depropanizedcompressed overhead stream and taking said third hydrocarbon stream fromsaid depropanized compressed overhead stream.
 16. The process of claim14 wherein the third hydrocarbon stream has more than 20 wt % olefins.17. A process for catalytic production of olefins comprising: contactinga first hydrocarbon stream and a first stream of fluid catalyst in afirst riser to produce a first cracked product stream and a spentcatalyst stream; separating said first cracked product stream in a maincolumn; obtaining a second hydrocarbon stream from an overhead line ofsaid main column system; contacting said second hydrocarbon stream witha second stream of fluid catalyst in a second riser to produce a secondcracked product stream and a first stream of cool catalyst; andobtaining a third hydrocarbon stream from said first cracked productstream or said second cracked product stream; and contacting said thirdhydrocarbon stream with a third stream of fluid catalyst in a thirdriser to produce a third cracked product stream and a second stream ofcool catalyst.
 18. The process of claim 17 wherein said secondhydrocarbon stream is more olefinic and/or more crackable than saidthird hydrocarbon stream.
 19. The process of claim 17 wherein the thirdhydrocarbon stream has more than 20 wt % olefins.
 20. The process ofclaim 17 further comprising operating the third riser at a greaterseverity than the second riser.